example(uffd): dont panic if read(2) from uffd returns -EAGAIN

Started seeing the below failure in test_population_latency:

thread 'main' panicked at .../uffd/fault_all_handler.rs:41:18:
uffd_msg not ready
note: run with `RUST_BACKTRACE=1` environment variable to display a
backtrace

I am not entierly sure how this can happen, because the read from the
uffd is supposed to be blocking, but maybe it's a weird interaction
with the fault-all behavior (e.g. there was a uffd event queues, but
because we faulted everything it got cancelled again?), so let's just
try going back to read(2) if we dont read anything.

Signed-off-by: Patrick Roy <[email protected]>

Author: roypat

src/firecracker/examples/uffd/fault_all_handler.rs

@@ -35,10 +35,9 @@ fn main() {
     runtime.run(
         |uffd_handler: &mut UffdHandler| {
             // Read an event from the userfaultfd.
-            let event = uffd_handler
-                .read_event()
-                .expect("Failed to read uffd_msg")
-                .expect("uffd_msg not ready");
+            let Some(event) = uffd_handler.read_event().expect("Failed to read uffd_msg") else {
+                return;
+            };
 
             if let userfaultfd::Event::Pagefault { addr, .. } = event {
                 let bit =